1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier and a printer for performing image formation by using image forming processes including a process by which an image bearing member such as an elecrophotographic photosensitive member or an electrostatic recording dielectric member is charged with a predetermined polarity and a predetermined potential.
2. Description of the Related Art
Conventionally, an image forming apparatus using an electrophotographic method often employs a process cartridge system. Such an image forming apparatus includes a cartridge that is attachable to and detachable from an apparatus main body thereof. The cartridge is integrally formed including a rotatable photosensitive member and a process unit acting upon the photosensitive member.
The use of the process cartridge system enables a user to perform maintenance work on the apparatus, which eliminates the necessity for maintenance work by a service person. This can markedly enhance operability. Thus, the process cartridge system has been widely used in the electrophotographic image forming apparatuses.
In the image forming apparatus such as a laser beam printer and a copier employing the electrophotographic method, a photosensitive member uniformly charged by a charging roller is first irradiated with light (e.g., laser beam) corresponding to image information to form an electrostatic latent image thereon. Subsequently, a developing device supplies developer (toner) to visualize the electrostatic latent image as a developed image (a toner image). The developed image is transferred from the photosensitive member onto a recording material such as a sheet, so that the image is formed on the recording material and then output.
The image forming apparatus employing such a transfer method may include a cleaner (a cleaning device) that removes residual transfer developer, which is remaining on the photosensitive member without being transferred to the recording material, from a surface of the photosensitive member. In such a case, the residual transfer developer is treated as waste developer. However, it is desired that the waste developer should not be generated from an environmental protection standpoint. Accordingly, there is an image forming apparatus in which a developing device performs “development and cleaning at the same time” without using a cleaner. More specifically, after a developed image is transferred from a photosensitive member, the developing device removes a residual transfer developer from a surface of the photosensitive member. The developing device collects the residual transfer developer, and reuses the collected developer. In other words, the image forming apparatus employs a developer recycling process.
The term “development and cleaning at the same time” used herein indicates a method for collecting developer remaining on a photosensitive member without being transferred to a recording material by using a residual toner collection bias when a next or subsequent development process is performed. More specifically, the method uses a fogging prevention potential difference Vback that is a potential difference between a direct current voltage applied to the developing device and a surface potential of the photosensitive member. According to this method, the developing device can collect the residual transfer developer, and reuse the collected residual transfer developer in the next or subsequent development process. This can eliminate waste developer and save maintenance work. Moreover, a so-called cleanerless image forming apparatus in which the residual transfer developer is collected by the developing device has an advantage in size. More specifically, since the image forming apparatus does not need to have the cleaning device, there is an advantage that size of the image forming apparatus can be markedly reduced.
In an image forming apparatus using a contact-type charging member (a charging roller), the charging member that contacts an image bearing member may pick up residual developer from a surface of the image bearing member. This may cause the residual developer to adhere to a surface of the charging member. Consequently, in a case where printing is repeatedly performed (durability), there is a possibility that chargeability may deteriorate due to an adhesion amount of the developer to the charging member.
Particularly, when the cleanerless image forming apparatus forms an image, residual transfer developer is liable to enter into a charging nip serving as a contact portion between the contact-type charging member and the image bearing member. This causes the developer to adhere to a surface of the contact-type charging member. In a case where there is developer on the contact-type charging member, a charging potential of the image bearing member varies depending on a developer adhesion amount. Such a phenomenon may appear as fluctuations in halftone image density.
Japanese Patent Application Laid-Open No. 2002-207353 discusses a method for stabilizing a charging potential while reducing fluctuations in chargeability. According to the method, charging accelerator and developer are mixed and applied beforehand to a new charging member. This reduces fluctuations in chargeability and stabilizes a charging potential when use of an apparatus is in initial state.
As for such a method, however, the admixture needs to be applied when the charging member is produced. This may lower productivity. In addition, there is a possibility that the applied admixture may drop onto an image forming apparatus or inside an apparatus body due to, for example, vibration during transport.
Accordingly, there has been a demand to reduce the fluctuations in the chargeability in the initial state without application of the admixture beforehand when the image forming apparatus is produced.